Some of my friends and relatives don't believe in climate change, so I regularly get emails containing evidence that climate change isn't real. The "evidence" contained in these emails usually falls into one of two categories.Some of it is just bogus — "facts" that are not true or that have been willfully misinterpreted or distorted. There isn't much to say about this, except shame on those who spread such baloney.

The other thing I find when I click on the links in these emails are reports of studies that seem to contradict the dogma of climate change. I'm sure you've seen these — studies showing that an ice sheet is expanding, or that temperatures have been dropping at a weather station somewhere.

Indeed, such studies do exist, and sometimes they are scientifically valid. They do not necessarily threaten ideas about climate change, however, a point that often is misunderstood by non-scientists. Instead of staking everything on any single study, scientists consider the weight of all evidence when reaching their conclusions, whether about climate change or any other subject.

A new study in Geophysical Research Letters by a team led by NOAA's David Anderson illustrates this point nicely. There has been some concern about whether direct measurements of temperatures (using thermometers at weather stations) fairly represent global warming over the past century or two, because these stations are not evenly distributed across the planet, often are near cities (which are heat islands) and so on. Of course, climate scientists are pretty smart, so they have tried to correct the temperature record for these biases, and it is this corrected temperature record that serves as one of the foundations for the conclusion that the Earth has been warming recently.

But what if this corrected temperature record is somehow wrong? Is there some other way we can tell if the planet has been warming?

It turns out that the chemical makeup of structures such as coral reefs, glacial ice and mussel shells records the temperature at which the structure was built. Furthermore, the growth rates of living things and the timing of plant leaf-out also can be used to infer past temperatures. One of the advantages of using these indirect records of past temperatures is that they can be collected around the world, not just near cities, getting around one of the possible problems with direct measurements of temperature.

Anderson and his team combed through the scientific literature and found 173 separate studies where temperatures since 1880 had been estimated by these indirect methods. Reassuringly, they found that estimates of global temperature using these indirect measures agreed very well with the corrected temperature records measured at weather stations. This study provides yet more evidence that global warming can’t be written off simply as a result of biased measurements by climate scientists.

One very interesting finding of the Anderson study is that 13 of the 173 studies they reviewed found evidence of cooling rather than warming. Should we interpret these studies to mean that the global climate has not been warming?

Such a radical interpretation is hard to defend. Remember that Anderson’s team found that the average of the 173 studies clearly showed that temperatures have been increasing around the globe since 1880. Furthermore, let’s not forget that the direct measurement of temperatures, made at numerous places around the globe, and carefully vetted by countless climate scientists, show this same warming trend. And other kinds of evidence, not even considered by Anderson’s group, like shorter periods of ice cover and shifts in distributions of plants and animals towards the poles and high elevations, also confirm the warming trend. I just don’t see how you could throw out all this evidence just because a few studies go against the trend.

Instead, it seems most logical to interpret these scattered contradictory results as a result of imperfect scientific methods being applied to a complicated natural world whose pieces do not always move in lockstep. These interesting contrary results certainly would repay closer investigation but do not invalidate the general conclusion that the Earth has been warming.

The next time you get an email claiming to invalidate some scientific claim, remember that one swallow does not make a summer. Consider all the evidence.